This invention relates generally to articles made from low-density structural materials and more particularly, to a method of fabricating a low density structural article having a low dielectric constant.
A number of aerospace and other structural applications require the use of non-metallic materials which are lightweight and have a low density along with good mechanical and structural properties. More recently, there is an increased need for such materials which also act as electromagnetic and particularly, radar absorbing or transparent materials. For example, radomes that house radar antennas must be lightweight, yet have sufficient structural strength and toughness to withstand the aerodynamic forces experienced during flight. The materials used to construct the radome must be of a sufficiently low dielectric so as not to interfere with the transmitted signals.
Syntactic foams are often used as a lightweight material having good mechanical and structural properties as well as radar absorbing characteristics. These syntactic foams include open pores or cells that are formed by incorporating small diameter hollow objects or microballoons into a resin bonding agent which is then cured into a foam structure. Syntactic foams are generally structurally superior to blown foams, such as polyurethane foam and expanded polystyrene foam. In addition, the lower the density of the foam, the lower the dielectric. However, these syntactic foams generally do not have sufficient mechanical properties, such as strength and outer surface durability, required for aerospace and related structural applications.
In an effort to improve the mechanical and other properties of these syntactic foams, an outer layer of a structural material such as an outer skin is often applied and bonded to the syntactic foam article or core. This outer skin generally improves the mechanical properties of the foam core adding structural strength and rigidity to the finished structural article. However, bonding or otherwise attaching an outer skin to the syntactic foam core has a number of drawbacks and problems.
Typically, a resin adhesive is used to secure the outer skin to the syntactic foam core. Adhesives are also used to secure articles made from the syntactic foam core to other structures. The adhesive is applied to the desired surface of the foam core using a variety of methods and techniques, all of which are labor intensive and time consuming. In addition, the adhesive typically has poor electrical properties relative to the syntactic foam. Specifically, applying an adhesive to the syntactic foam can significantly raise the dielectric or degrades the overall electrical properties, such as the radar absorbing characteristics of the finished structural article.
Prior to applying an adhesive, the syntactic foam core must be sealed. Specifically, any surface on the foam to which a resin type adhesive, a bonding agent or most any other substance will be applied must, be pre-sealed. Pre-sealing prevents the leaching or other draining of the resin matrix from the adhesive or other applied substance into the cells or open pores of the foam core. However, pre-sealing the syntactic foam core is also labor intensive, time consuming, expensive, and generally degrades the radar absorbing characteristics of the finished structural article.
One method of sealing a porous foam, such as a blown foam, is described in U.S. Pat. Ser. No. 5,472,541, issued to Simmons et al. This patent discloses a method for applying a thin coating of a powdered adhesive to a porous foam article to form a sealed layer. After being coated, the foam article is placed on an adhesive coated belt. The belt is then moved into a heating zone where the coating of the adhesive is fully melted to the lower surface of the foam article.
Methods of sealing and applying an adhesive to a surface of a syntactic foam article or core include coating the desired surface of the syntactic foam article with a mastic like sealer. This sealer is generally applied manually, by hand application. After the sealer has been applied, the adhesive can be applied on top of the sealer. These methods, including the applied materials, have many disadvantages. For example, the currently used sealing and adhesive materials have significantly higher dielectric constants than the syntactic foam core. This degrades the radar absorbing characteristics of the overall structural article. The application of both the sealer and the adhesive is time consuming, expensive and the rigors of the manual application process has the potential to crush or otherwise damage the syntactic foam core. It is also difficult to control the thickness of the sealer and adhesive using the current materials and methods.
There is thus a need for a structural article having a syntactic foam core with a structural outer skin and which has improved radar absorbing characteristics. There is also a need for a method of fabricating a three dimensional structural article having improved radar absorbing characteristics.
The present invention overcomes these problems of the past by providing a lightweight three dimensional structural article, made from a plurality of materials, and which has excellent mechanical properties such as strength and durability as well as good radar absorbing characteristics. The structural article is made from a structural foam core having a structural laminate outer skin and can be formed into most any shape or configuration. In addition, the materials and particularly, the resin systems of the various materials may be specifically formulated and adapted to provide a finished structural article having a desired density and having desired radar absorbing characteristics. By using materials specifically formulated for their improved radar absorbing characteristics and mechanical properties, the finished article retains overall low radar reflectivity characteristics.
The present invention also satisfies the need for a method of fabricating such a lightweight three dimensional article. The method utilizes a syntactic sealing adhesive foam which is compatible with the structural outer skin and which seals the structural foam core. The syntactic foam sealing adhesive material, has a low dielectric constant for improved radar absorbing characteristics. In addition, the materials comprising the syntactic foam sealing film can be formulated to vary density and dielectric constant and thus, modify the properties of the finished structural article. The sealing adhesive film allows the outer skin to be applied to the structural foam core and cured to form the final structural article.
The present invention is generally directed to a three dimensional structural article having a desired configuration and improved radar absorbing characteristics. The article is made from a plurality of low density materials, each having a low dielectric constant. Particularly, the structural article comprises a laminate having a three dimensional syntactic foam core, a layer of a syntactic sealing adhesive film and a structural outer skin.
The syntactic foam core is formed substantially into the desired shape or configuration and includes at least one exposed surface. The syntactic foam core, and particularly, the exposed surface is formed slightly smaller than the final desired configuration to accommodate the thickness of the sealing adhesive film and the outer skin. The outer skin covers the exposed surface and is sized to the desired configuration. The outer skin provides a structurally strong article with a durable surface.
A layer of the foam sealing adhesive film is disposed between the exposed surface on the syntactic foam core and the outer skin. The sealing adhesive film is preferably a prefabricated sheet of a syntactic foam sealing adhesive film having a plurality of microballoons intermixed with a resin. The prefabricated sheet is xe2x80x9cBxe2x80x9d stage cured and cut or otherwise sized to fit over the exposed surface. The sealing foam seals the exposed surface on the foam core and adheres the foam core to the outer skin.
Methods for making a three dimensional article having a desired shape or configuration from a plurality of materials, each having a low density and low radar reflectivity characteristics, for example, the presently described three dimensional article is included within the scope of the present invention. The method includes the steps of providing a structural thermoplastic syntactic foam core. The syntactic foam core is provided or otherwise formed into substantially the desired configuration and includes at least one exposed surface. Particularly, the foam core is formed slightly undersized from the desired configuration to accommodate the thickness of the additional materials required to fabricate the article.
An outer skin is also provided. The outer skin comprises a plurality of structural fibers intermixed with a resin matrix. The outer skin is formed or applied such that it conforms to the exposed surface on the foam core.
The method also includes the steps of providing a layer of a syntactic foam sealing adhesive film. The sealing adhesive film has a dielectric constant of less than 2.5 to promote the low radar reflectivity characteristics of the finished article. The sealing adhesive film is adapted for sealing the syntactic foam core and for adhering the foam core with the provided outer skin.
The formed foam core is supported such that the exposed surface is not interfered with or obstructed. A layer of the sealing adhesive film is placed over the exposed surface. The outer skin is pre-cured and then placed on the foam core such that the layer of sealing adhesive film is compressed and sandwiched between the outer skin and the exposed surface. This forms the desired configuration of the structural article, but in a pre-cured state. The pre-cured article is then secondarily bonded to form the three dimensional structural article. This step of secondarily bonding cures the sealing adhesive film and the pre-cured outer skin such that the outer skin is cured into a structural outer skin and is also secured to the foam core.
In another aspect of the present invention, the step of providing an outer skin comprises providing a layer of an uncured prepreg material. The uncured or partially cured prepreg outer skin is placed over the sealing adhesive film which was previously placed over the exposed surface of the foam core. In this aspect of the present invention, the prepreg outer skin and the sealing adhesive film are co-cured.
This invention, together with the additional features and advantages thereof, which is only summarized in the foregoing passages, will become more apparent to those of skill in the art upon reading the description of the preferred embodiments, which follows in the specification, taken together with the following drawings.